The present invention relates generally to multilumen catheters. More particularly, the present invention relates to multilumen catheters having proximal IV tubes welded directly with the walls of the catheter's lumens to create a continuous and integral bond therebetween. This invention is particularly, though not exclusively, useful as a multilumen catheter having relatively thick walled IV tubes of generally circular cross-section welded to the catheter lumens of generally noncircular cross-section.